It is important to predict the lifetime of a storage battery for the maintenance of an apparatus that incorporates a backup storage battery such as an uninterruptible power supply (UPS). While corrosion of the hydrogen-absorbing alloy used to form the negative electrode is a major cause of deterioration in the lifetime of a nickel-hydrogen storage battery, other factors may also contribute to the deterioration such as the operating temperature, the number of discharge cycles, the elapsed time and the load power applied during discharge. The variety of factors that affect the lifetime make it difficult to precisely predict the lifetime of a storage battery in use.
It has been proposed to use an increase in internal resistance in the latter stage of life and/or change in voltage during discharge as the parameter for estimating the capacity and lifetime of a nickel-hydrogen storage battery. For example, an apparatus disclosed in Japanese Unexamined Patent Publication No. 8-138759 estimates the deterioration from the distribution of discharge voltages corresponding to a plurality of discharge currents and calculating the gradient thereof. An apparatus disclosed in Japanese Unexamined Patent Publication No. 2000-215923 estimates the deterioration by comparing the internal resistance and battery voltage measured during discharging with the initial values thereof. These lifetime predicting methods are based on the correlation between the internal resistance of the storage battery and the resultant change in voltage and the lifetime of the storage battery, and are useful due to the ability to predict the lifetime in a short period of time.
Meanwhile such a method has been proposed that predicts the lifetime of a storage battery from a remaining lifetime value that is the difference between an expected lifetime value computed from the power applied to the load during discharge and a diminution in lifetime calculated as a linear function of the number of discharge cycles (refer to, for example, Japanese Unexamined Patent Publication No. 2000-243459). This method enables prediction of the lifetime while correcting a highly accurate expected lifetime value without the need to forcibly discharge the storage battery, and is therefore useful for lead-acid storage batteries.
However, the methods disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-138759 and Japanese Unexamined Patent Publication (Kokai) No. 2000-215923, cannot predict the lifetime unless the internal resistance increases to some extent and, moreover, do not take into account such factors as rate of discharge and battery temperature that contribute to the diminution of lifetime. The method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2000-243459 has such a problem that, since the equation used to estimate the lifetime is not a linear equation of the number of discharge cycles because the nickel-hydrogen storage battery has an intrinsic deterioration behavior (corrosion of hydrogen-absorbing alloy used in the negative electrode), the estimate of the remaining lifetime value deviates significantly from the actual value.